In recent years, “gels” as soft materials are expected to be applied for industrial purposes in a wide range of fields such as food products, cosmetics, sport shoes, and chromatography.
Many of them, however, are natural gels, such as agar, gelatin, and carrageenan, and their function has limits. It is also difficult to further impart an optimal function only by chemical modification thereof.
Under the circumstances, recently, basic research and application development have been actively conducted on synthetic gels artificially mimicking the functions of natural gels.
A known example of such synthetic gelling agents is a hydrogel-forming agent using a polymer such as polyacrylic acid. However, hydrogels produced with such a polymer gelling agent are irreversible chemical gels that cannot return to the original water once formed, and it is impossible to control the physical properties of the formed gels, such as the hardness and thermal stability of the gels.
Furthermore, recently, there are also proposed some synthetic hydrogel-forming agents using biodegradable or biocompatible molecular structure units (for example, see Patent Documents 1 to 3 below). Each of these agents requires a multi-step process of synthetic steps and separation operation, and thus presents a significant challenge to large-scale synthesis for practical use.
Furthermore, conventional synthetic hydrogel-forming agents and natural hydrogel-forming agents (agar) also have a problem in which adaptable aqueous solutions are limited to those with acidity close to neutral, because they contain, in their molecular structure, an acetal bond or ester bond which is unstable under acidic conditions.
In addition, hydrogels produced with these gelling agents show slow equilibrium between a quasi-solid state under no-mechanical load conditions and a quasi-liquid state produced under high distortion conditions. Once the quasi-solid structure of such hydrogels is collapsed by mechanical impact, it generally takes a long period of time, for example, the order of several hours to several days for gelatin, to recover the structure. This significantly limits the applicability of the gels.
In order to solve this problem, there is proposed a copolymer macromolecular gelling agent having a charge in its side chain (for example, see Non-Patent Document 1 below). However, it is complex in structure and synthesis and thus cannot be popular.
There is also a demand for artificial gelling agents adaptable to not only water but also a wide variety of solvents. For example, since ionic liquids are non-volatile and highly ion-conductive, gels thereof made by gelling those are expected to find applications in the field of cells, such as solid (quasi-solid) electrolytes for secondary cells and sensors, and expected to be applied to organic synthesis reactions in gelled liquids.
Several types of low-molecular compounds capable of gelling such ionic liquids have been synthesized and developed in the past (for example, see Patent Document 4 and Non-Patent Document 2 below). Each of these compounds has a complicated molecular structure and thus requires a multi-step synthetic process and separation/purification operation. They are also known to reduce their ion conductivity (electrical conductivity) due to an increase in viscosity after the gelation, which is a challenge to be overcome as soon as possible.
There is also proposed another synthetic polymer gelling agent (for example, see Patent Document 5 below). However, a relatively large amount of such a gelling agent is necessary for gelation of ionic liquids, and there is a problem in which since an additional solvent such as water and acetone is often used for the gelation, the process of forming a gel consisting of a pure ionic liquid has to undergo high temperature drying for removing such an auxiliary solvent.
Furthermore, concerning polymer gelling agents, a method is developed which uses a thermally irreversible chemical gel (see Non-Patent Documents 3 and 4 below). This method includes mixing an ionic liquid electrolyte, a gelling agent, and a crosslinking agent, to form a gel electrolyte precursor, injecting the precursor into a cell, and then heating the precursor, to cause gelation in the cell. However, the gel has a chemically-bonded, three-dimensional, network structure and thus does not return to a solution state even at high temperature.
On the other hand, carbon nanotubes are attracting attention as useful materials for nanotechnology and expected to be applied in a wide range of fields such as transistors, electron emission electrodes, fuel cell electrodes, and scanning microscope chips. When they are purified or prepared for applications as materials for the applications, it is necessary to prepare an easily handleable carbon nanotube solution or dispersion (dispersed liquid) or a gel containing it.
Thus, there is proposed a method for making hydrophobic carbon nanotubes soluble in a solvent, which includes adding a dispersing agent (generally an amphiphilic surfactant) to form a dispersion liquid (for example, see Patent Document 6 and Non-Patent Document 3). Under the present circumstances, however, investigations for further improvements are still being carried out.
Hitherto, Patent Document 7 and Non-Patent Document 4 listed below are known to disclose carbon nanotube-containing gel materials. In this technique, however, it is necessary to use a special solvent of an ionic liquid, and thus it is difficult to prepare a gel with a low environmental-load common solvent such as water.
As described above, conventional gelling agents only have a gelling function. At present, there has been developed no artificial gelling agent having another function such as a dispersing function in combination with a gelling function, and for example being capable of dispersing single-walled carbon nanotubes in a medium of water and forming a gel at the same time.
Patent Document 1: JP-A-2003-327949
Patent Document 2: JP-A-2003-49154
Patent Document 3: JP-A-2003-55642
Patent Document 4: JP-A-2002-3478
Patent Document 5: JP-2003-257240
Patent Document 6: JP-A-2003-238126
Patent Document 7: JP-A-2004-142972
Non-Patent Document 1: Nature, Vol. 417, p. 424 (2002).
Non-Patent Document 2: Chem. Commun. 2002, p. 374.
Non-Patent Document 3: Science, Vol. 297, p. 593 (2002).
Non-Patent Document 4: Science, Vol. 300, p. 2072 (2003).